California Governor Gavin Newsom called the death penalty morally wrong, discriminatory and a failure today. He then signed an executive order placing a moratorium on capital punishment.

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GAVIN NEWSOM: I cannot sign off on executing hundreds and hundreds of human beings knowing - knowing that among them will be innocent human beings.

CORNISH: The move grants a temporary stay for the 737 inmates who make up the nation's largest death row population. Death penalty supporters say the Democratic governor's actions go against the will of Californians. Opponents hope the move will bolster other state-level efforts underway to suspend or outlaw capital punishment.

NPR's Eric Westervelt has been following the developments from San Francisco. And, Eric, first tell us what the governor's move does for those who are currently on death row in the state.

ERIC WESTERVELT, BYLINE: Yeah, hi, Audie. Those more than 700 inmates here are still under a death sentence, but they've been granted reprieves under the governor's authority as the executive of the state. So they're not at risk of execution as long as Gavin Newsom is governor. The move also closes the new death chamber at San Quentin Prison. And the governor also did away with the state's approved way or method of carrying out executions - the lethal injection protocols, as they're called. So with those three moves, the governor has formally suspended the death penalty in California.

But it's sure to face challenges. Those are definitely coming from the courts and from folks in the courts. I mean, death penalty proponents aren't happy. President Trump today tweeted, friends and families of the always-forgotten victims are not thrilled, and neither am I.

CORNISH: The state hasn't carried out an execution in more than a decade. You talked about legal challenges. The death penalty in California was already stalled by court challenges. So is this largely a symbolic move?

WESTERVELT: I think it is, but I think symbolism matters when you're talking about the nation's most populous state. I mean, it's the largest death row in America. And now California joins three other states that have imposed a death penalty moratorium. So now, you know, fully one-third of all prisoners on death row in the U.S. are in states in which governors have said, you know, they won't be executed. So opponents of the death penalty say, look; that's huge.

And there's a practical side here, too, Audie, beyond symbolism. And California, like several other states, just doesn't have the availability of the lethal injection drugs to carry out an execution even if it wanted to. I mean, there's this enormous nationwide controversy over what drugs to use at all to carry out these killings.

CORNISH: You mentioned other states that have enacted moratoriums or have outlawed executions. Are we looking at a trend?

WESTERVELT: Big trend - New York state, Washington, Colorado, New Hampshire, Oregon - lawmakers there are taking steps to limit the death penalty. Those are in action. But, you know, the momentum, death penalty opponents say, is on their side. Twenty states have now done away with the death penalty altogether, Audie, and another four have imposed moratoriums that, you know, effectively make executions impossible.

So, you know, it makes almost half the country, you know, that doesn't have the death penalty as a punishment or said it just won't carry out the executions - and several states that have a death penalty on the books aren't using them. Seven states, Audie, haven't carried out any execution in more than a decade.

CORNISH: What are some of the other factors driving this?

WESTERVELT: I mean, as Governor Newsom mentioned in his announcement today in California, studies show the death penalty discriminates against the mentally ill. It discriminates against people of color. African-Americans make up barely 13 percent of the U.S. population, but they're 34 percent of inmates on death row. And I think also importantly, I mean, the number of wrongful convictions - 164 people have been released from death row nationwide. That's huge. Those exonerations have had a big impact on policymakers and probably the public as well who say that's just an unacceptable error rate when someone's life is at stake.